It is often desirable to mitigate a component or components from a fluid stream, particularly where the fluid stream is a gaseous emission from a process which contains compounds which are harmful to humans and/or the environment. Examples of such compounds include volatile hydrocarbons such as methane (CH4). Fugitive methane emissions occur from a variety of sources including coal, oil and gas production, transport, mining, agriculture, waste disposal, livestock, waste water treatment and land use (forestry).
Current data indicates that, of the anthropogenic gases that contribute to global warming, methane (CH4) is the most significant after carbon dioxide (CO2). On a unit basis, CH4 is estimated to be 25 times more potent at trapping heat in the atmosphere than CO2 over a 100 year period. While methane originates from several sources, fugitive CH4 emissions from coal mines represent approximately 8% of the world's anthropogenic CH4, and contribute roughly 17% to anthropogenic emissions. Coal mine methane (CMM) is not only a greenhouse gas but also represents a significant wasted energy resource which, under certain conditions, could be effectively used for electrical generation, heating or chemical manufacturing feedstock. It was estimated that about 28 billion m3 of CH4 (equivalent to 420 million tonnes of CO2) are emitted annually to the atmosphere from coal mining activities around the world in 2010.
Depending on coal mine site specifications, approximately 50-85% of all coal mining related methane is emitted to the atmosphere in mine ventilation air. The development of technologies for ventilation air methane (VAM) capture, mitigation and utilisation are on-going challenges because the ventilation air volume flow rate is large and the methane concentration is dilute and variable. A typical gassy mine in Australia produces ventilation air at a rate of approximately 120 to 600 m3/s with methane concentrations of 0.3-1%.
Existing technologies used to mitigate methane from mine ventilation air include a range of techniques such as techniques based on methane oxidation and adsorption. In methane oxidation systems, a gaseous feed containing methane is introduced to a combustion module where the gaseous feed is heated. When the gaseous feed reaches the auto-ignition temperature of methane, oxidation of the methane takes place. The reaction can be classified as either thermal oxidation occurring at temperatures in the order of 850-1300° C., or catalytic oxidation occurring at temperatures in the order of 450-800° C.
For mine site application, ventilation air is conveyed to VAM combustion modules through a ducting system (either unenclosed or enclosed) from the mine ventilation air shaft. Enclosed ducting is required to capture the full ventilation air flow. Whether ducting is unenclosed or enclosed, an unplanned event, e.g. the release of a pocket of higher concentration methane into the ventilation air, could result in an explosive mix of methane which is directly ducted to a potential ignition source in the methane combustion modules.
For combustion to occur, the level of a combustible gas must be between its Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL). The upper and lower explosive limits are defined as the lowest concentration (by percentage) of a gas or vapour in air that is capable of producing a flash of fire in the presence of an ignition source. For methane and air mixtures, the LEL is 5% CH4 and the UEL is 15% CH4. In the event that a combustible gas at levels in the LEL to UEL range comes into contact with an ignition source, combustion may occur. In general, there are two important regimes of combustion: deflagration and detonation.
Deflagration is characterised by a subsonic flame front velocity. The main mechanism of combustion propagation is of a flame that propagates due to heat transfer effects. Detonation is characterised by a supersonic flame front velocity which propagates due to a powerful pressure wave that compresses the unburnt gas ahead of the wave to a temperature above the auto-ignition temperature. The effects of detonation on a confined system can be devastating.
In confined systems such as ducting, obstacles in the flame path such as elbows, sensors and other attachments can cause turbulence in the flame, thus accelerating a subsonic flame (deflagration) to a supersonic speeds (detonation). The transition from a deflagration type of combustion to a detonation type of combustion is known as the deflagration to detonation transition (DDT).
Due to the presence of combustible methane in ventilation air, when any VAM technologies with a potential ignition source are commercially implemented at mines, a major concern faced by the coal industry is the safety of connecting the VAM combustion modules to the mine ventilation air shaft. Existing ducting systems for commercial scale VAM combustion modules operating at coal mines rely on prevention methods utilising monitoring and mechanically operated safety features. However, these existing prevention measures can be unsuccessful as the failure of any one of the monitoring and mechanically operated safety features can render the entire fire prevention system useless. Furthermore, faulty prevention measures can act as an ignition source in the system which could result in the ignition of the combustible component they were aimed at preventing.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.